Conventionally, in a manufacturing process of automobiles, the bores of the cylinder block of an engine are machine processed, after which the cylinder head, crank case, etc, are mated to the cylinder block.
Herein, since the pistons accommodated in the bores are round cross-sectional shapes, the cross-sectional shapes of the bores are machine processed so as to become in a state of nearly a circle.
However, even if the bores of a cylinder block were processed to be a round cross-sectional shape, the shape of the bores would deform when the cylinder head, crank case, etc. are mated. When a bore deforms in such a way, it is a primary cause increasing the sliding resistance between the bore and piston during operation of the engine, and the engine may not be able to realize the desired performance.
Therefore, when processing the bores of a cylinder block, a dummy head replicating a cylinder head is attached and processing on the bore is performed, and when the processing on the bore finishes, the dummy head is detached.
However, when carrying out attachment and detachment of the dummy head and the like every time bore processing the cylinder block, there is a problem in that productivity declines greatly.
In order to solve this problem, the following method has been proposed (refer to Patent Document 1).
Specifically, first, a dummy head is mounted to the cylinder block, and the bores are processed into a round cross-sectional shape by a machine tool.
Next, the dummy head is detached from the cylinder block. When this is done, the shape of the bore deforms and becomes a non-round cross-sectional shape since the stress from mating of the dummy head is eliminated. The overall shape of the bore after this dummy head has been detached is measured to generate NC data.
Thereafter, boring processing of the cylinder block is performed based on the NC data thus generated, without mounting a dummy head. If done in this way, the bore will become a round shape when the cylinder head is mounted, even if the bore is processed without mating a dummy head to the cylinder block.
In addition, in Patent Document 1, in order to form a bore with a cross-sectional shape that is non-round as described above, the following processing head of a machine tool has been proposed.
Specifically, the processing head includes a spindle shaft of a cylindrical shape, a shaft stored inside of this spindle shaft to freely advance and retract in an axial direction, and a boring bit that is provided at a tip end of the spindle shaft to be able to project and retract in a centrifugal direction. A taper portion abutting a base end side of the bit and a grindstone is provided in a tip end of the shaft.
According to this machine tool, by causing the shaft to advance or retract, the base-end sides of the bit and the grindstone are pressed by the taper portion, and thus the projecting amount of these bits can be adjusted. Therefore, the bore can be processed to a desired cross-sectional shape by adjusting the projecting amounts of the bit and grindstone while causing the spindle shaft to rotate.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2007-313619